Blood pressure sensor apparatus

ABSTRACT

An implanted sensor for measuring pressure in a conduit through a wall has a main body and a probe. The main body includes an implant inductor. The probe has capacitor electronically connected to the implant inductor. The probe is adapted to fit through the wall so that the capacitor can sense pressure in the conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application for a utility patent claims the benefit of U.S.Provisional Application No. 60/458,660, filed Mar. 28, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] This invention relates generally to a blood pressure sensorapparatus, and more particularly to a blood pressure sensor apparatusthat can be implanted into a patient and used to regularly report theblood pressure of the patient.

[0005] 2. Description of Related Art

[0006] The monitoring of blood pressure by caregivers has become awell-characterized biomonitoring tool. Hypertension, hypotension, shockand circadian rhythm are some examples of conditions monitored via bloodpressure. In most cases, the usage of a sphygmomanometer and a pressurecuff suffice. But in cases where long-term, mobile, non-tethered, and/orphysician-free patient monitoring is required, a more elaborate andimplantable system may be needed.

[0007] The foremost requirement for implantation is the size of thedevice. The implant should not impart any physiological disturbance norshould it present any substantial inconvenience. Furthermore, the devicemay only protrude into a blood vessel a very small amount, because theintroduction of a significant disturbance into a blood vessel can causehealth problems.

[0008] Supplying power to the device and rate of power consumption arealso important factors because battery size and replacement are criticallimiting factors to the miniaturization and operation of the device.Finally, a means of transmitting the signal is an integral part of theimplant as well as a technique to encapsulate the entire device for thebilateral protection of the physiology and the implant.

SUMMARY OF THE INVENTION

[0009] The present invention teaches certain benefits in constructionand use which give rise to the objectives described below.

[0010] The present invention provides an implanted sensor for measuringpressure in a conduit through a wall. The implanted sensor includes amain body and a probe. The main body includes an implant inductor. Theprobe has capacitor electronically connected to the implant inductor.The probe is adapted to fit through the wall so that the capacitor cansense pressure in the conduit.

[0011] A primary objective of the present invention is to provide animplanted sensor having advantages not taught by the prior art.

[0012] Another objective is to provide an implanted sensor that canreadily be positioned outside of a conduit such as a blood vesselwithout undue trauma to the patient.

[0013] Another objective is to provide an implanted sensor that includesa probe that can be positioned through the blood vessel so that bloodflow within the blood vessel is not significantly impeded or disrupted.

[0014] A further objective is to provide an implanted sensor that can beinstalled in a single procedure and then take continuous blood pressuremeasurements without further surgical procedures being required.

[0015] Other features and advantages of the present invention willbecome apparent from the following more detailed description, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The accompanying drawings illustrate the present invention. Insuch drawings:

[0017]FIG. 1 is a perspective view of one embodiment of a blood pressuresensor apparatus;

[0018]FIG. 2 is a sectional view thereof taken along line 2-2 in FIG. 1;

[0019]FIG. 3 is a block diagram thereof;

[0020]FIG. 4 is a bottom perspective view of an implanted sensor;

[0021]FIG. 5 is a side elevational view thereof, a portion of theimplanted sensor being shown broken away to illustrate first and secondelectrodes;

[0022]FIG. 6 is a top perspective view of the implanted sensorillustrating a plurality of bores in a top surface of the implantedsensor;

[0023]FIG. 7 is a perspective view of the blood pressure sensorapparatus transmitting data to a personal transmitter/receiver that isoperatively attached to a computer; and

[0024]FIG. 8 is a perspective view of the blood pressure sensorapparatus transmitting data through a cellular transmitter/receiver to adata center.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The above-described drawing figures illustrate the invention, ablood pressure sensor apparatus 10 for periodically measuring the bloodpressure of a patient.

[0026] As shown in FIGS. 1-2, the blood pressure sensor apparatus 10includes an implanted sensor 20 and an external reader 30. The implantedsensor 20 is adapted to be implanted in the patient for sensing theblood pressure. The external reader 30 is adapted to be positionedadjacent the implanted sensor 20, outside the body of the patient, andinductively coupled to the implanted sensor 20 to periodically read theblood pressure of the patient.

[0027] In the preferred embodiment, the external reader 30 is awristwatch that can be conveniently worn by the user around his or herwrist. However, in alternative embodiments, the external reader 30 couldbe shaped to be worn around any portion of the body that is suitable forthe implanted sensor 20. While it is currently preferred that theexternal reader 30 be adapted to be worn for significant periods oftime, the external reader 30 could also be a hand-held scanner that isnot worn, but is periodically positioned adjacent the patient to takeblood pressure readings.

[0028] While we discuss the use of the blood pressure sensor apparatus10 to measure the blood pressure of a patient, typically a human, theblood pressure sensor apparatus 10 can be used to measure the bloodpressure in any animals, or indeed any closed system that includes afluid flow whose pressure may be measured. Such alternative applicationsof the present apparatus should be considered within the scope ofprotection of the present patent.

[0029] As shown in FIG. 3, the implanted sensor 20 includes an implantcircuit 22 that includes a capacitor C electronically connected to animplant inductor L1. The external reader 30 includes an external circuit32 that includes a power supply 34 electronically coupled to an externalinductor L2.

[0030] The implanted sensor 20 further includes a means for determiningthe blood pressure at the capacitor C using the implant inductor L1 andthe external inductor L2. The means for determining the blood pressureincludes sweeping the external inductor L2 through a range offrequencies using an oscilloscope 38 and measuring a dip at a specificfrequency, the specific frequency being determined by the capacitance ofthe capacitor C, which in turn is determined by the blood pressureexerted against the capacitor C. The oscilloscope 38 is adapted toperform a “grid-dip” sweep wherein the external reader 30 sweeps througha range of frequencies until it reaches a point that resonates with theimplant circuit 22 and the oscilloscope measures a “dip.”. Since thefrequency of resonance will vary depending upon the capacitance of thecapacitor C, and thus the patient's blood pressure, it is possible tomeasure the blood pressure of the patient from the external reader 30with reference to a simple calibration table.

[0031] The implant circuit 22 also includes a means for reporting theresults of the “grid dip” sweep. In one embodiment, as shown in FIGS. 1and 3, the external reader 30 includes a display 40, such as an LCDscreen or similar feature, then enables the user to read the results ofthe measurements being taken. In this embodiment, the external circuit32 includes a processor 42, a memory 44, and a keypad 46 for enablingthe user to control the external reader 30. The inclusion of theseadditional elements enables the user to store multiple readings withinthe memory 44 for later review and/or download to a computer 52 usingtechniques well known in the art. Since the construction of such acircuit is well known to one skilled in the art, given the teachings ofthis invention, the specific construction of the external reader 30 isnot described in greater detail herein.

[0032] As shown in FIG. 3, the external reader 30 can also include atransmitter/receiver 48 for transmitting the measurements taken by theexternal reader 30. In one embodiment, shown in FIG. 7, thetransmitter/receiver 48 transmits data to a personaltransmitter/receiver 50 that is electronically connected to a computer52. Upon a query from the computer 52, which could be located in apatient's home or in a doctor's office, the transmitter/receiver 48 ofthe external reader 30 could transmit the readings that were takenpreviously and stored in the memory 44.

[0033] In another embodiment, shown in FIG. 8, the transmitter/receiver48 could transmit the data using cellular technology through a cellulartransmitter/receiver 54 to a data center 56 for collection, analysis,and reporting. Obviously, many equivalent communications systems couldbe used, including satellite or IR transmissions, communications througha global computer network such as the Internet®, or a local areanetwork. Any of these or similar reporting systems should be consideredwithin the scope of the present invention.

[0034] Of course, communications between the external reader 30 and thecomputer 52 or the data center 56 would be two-way, thereby enablingmany options in taking, reporting, and responding to blood pressuremeasurements. For example, if a patient's blood pressure were to get sohigh or so low as to threaten the health of the patient, and immediatewarning could be sent to the patient, as well as the patient's doctorand/or a local ambulance dispatcher. The blood pressure sensor apparatus10 could also be integrated with other systems, such as a medicationinjection device (not shown), that would automatically administertreatment in response to high or low blood pressure.

[0035] As shown in FIGS. 4-5, the implanted sensor 20 preferablyincludes main body 58 and a probe 62 that extends outwardly from themain body 58. The main body 58 includes the implant inductor L1 and anyother electronics or other useful structural features. In oneembodiment, the main body 58 is generally cylindrical and the conductivematerial that forms the implant inductor L1 formed in a coil around aperimeter 60. Due to the minimum size requirements of the implantedinductor L1, the main body 58 is adapted to remain outside the bloodvessel 12 of the patient, thereby minimizing the potentially harmfulimpact of the implanted sensor 20 on the blood flow of the patient.

[0036] The probe 62 is adapted to extend into the blood vessel 12 forthe purpose of measuring the pressure in the blood vessel 12. The probe62 must be small enough to prevent thrombosis or other healthcomplications in the patient. In the preferred embodiment, the probe 62includes a neck portion 64 that extends outwardly to a head portion 66.The neck portion 64 is preferably cylindrical and includes an internalsaline chamber 68. The head portion 66 is shaped to penetrate throughand then lockingly engage the blood vessel 12. The head portion 66 ispreferably generally conical in shape. A terminus 70 of the head portion66 forms an aperture 72 that is covered with a flexible membrane 74. Theinternal saline chamber 68 is filled with saline or other biocompatiblefluid or equivalent material that is contained within the internalsaline chamber 68 by the flexible membrane 74.

[0037] The first electrode 26 forms the rear of the internal salinechamber 68 opposite the flexible membrane 74. The second electrode 28 ispositioned a suitable distance from the first electrode 26, separated bya gap 76 that is suitable to form the capacitor C. The first electrode26 is preferably a capacitive membrane formed of a highly doped siliconin conjunction with highly insulating support layers 80. The highlyinsulating support layers 80 are useful in limiting parasiticcapacitance, which may otherwise interfere with accurate pressuremeasurement. Those skilled in the art can devise many alternative formsof the first electrode 26, and such alternative structures should beconsidered within the scope of the present invention.

[0038] In operation, pressure from the blood vessel 12 causes adeflection of the flexible membrane 74, which is transmitted through thesaline in the internal saline chamber 68 to the capacitive membrane 26,which in turn is deflected. When the capacitive membrane 26 isdeflected, this changes the size of the gap 76 between the capacitivemembrane 26 and the second electrode 28, thereby altering thecapacitance of the capacitor C. Changes in the capacitance cause achange in the frequency at which the external reader 30 measures a “dip”in the oscilloscope 38, as described above.

[0039] The head portion 66, shown in FIGS. 4-5, is adapted to facilitatethe penetration of the probe 62 through a vessel of the patient so thatthe flexible membrane 74 is positioned inside the blood vessel 12, asshown in FIG. 2. The neck portion 64 is adapted to extend through theblood vessel 12 so that the main body 58 is located outside the bloodvessel 12, thereby minimizing any interference that the implanted sensor20 may cause within the blood vessel 12. The flexible membrane 74 isdisposed on an outside surface 78 of the implanted sensor 20 so that theflexible membrane 74 is exposed to the patient's blood once theimplanted sensor 20 has been implanted in the patient.

[0040] The implanted sensor 20, and the capacitive membrane 26, arepreferably constructed of silicon and formed using MEMS manufacturingtechniques known in the art. By utilizing MEMS construction techniques,the implanted sensor 20 can be made extremely small, thereby minimizingthe problems that can occur when a sensor is implanted in a patient'sbody. In one embodiment, as shown in FIG. 4, the implanted sensor 20 canbe coated with a biocompatible coating 82, or housed within a suitablybiocompatible structure, to prevent biocompatibility problems once theimplanted sensor 20 has been implanted into the patient.

[0041] The biocompatible coating 82 may also include embeddedanti-coagulants (not shown) that are released throughout the intendedlifetime of the sensing unit.

[0042] As shown in FIG. 6, an upper surface 84 of the implanted sensor20 may include a plurality of bores 86 or “bosses.” The plurality ofbores 86 function to increase the signal and improve the linearresponse. The plurality of bores 86 are preferably evenly spaced toincrease their effectiveness.

[0043] Alternative Sensor Means

[0044] While the inductor/capacitor system that is described herein iscurrently the preferred sensor means, alternative sensor means (notillustrated herein) could also be utilized. For example, the sensormeans could be provided by a piezoelectric sensor, a strain gauge, oranother sensor known to those skilled in the art.

[0045] These alternative sensor means could be powered by the inductorsystem described above, be miniature batteries operably installed in themain body 58 of the implanted sensor 20, or by a resonant circuit thatreceives power from an external signal and then returns a return signalthat reports a reading taken by the sensor means. Such alternativesshould be considered within the scope of the present invention.

[0046] Method of Implantation and Use

[0047] The implanted sensor 20 is preferably to be implanted in thedistal antebrachial region (forearm) adjacent the Ulnar or Radialarteries, since the thickness of integumentary tissues is relatively andconsistently thin across this portion of the body. This site will alsopermit for easy placement of the external reader 30, in the embodimentof a wristwatch. Of course, those skilled in the art could devisealternative locations for the implantation and monitoring of theimplanted sensor 20, and placement in an alternative location should beconsidered within the scope of the present invention.

[0048] The implanted sensor 20 preferably utilizes the passive systemdescribed above to eliminating any in-vivo power source requirement. Thecapacitive sensor system described above measures blood pressure bymeasuring the deflection of the capacitive membrane 26 that provides oneelectrode of a capacitive pair. The pressure sensor capacitance is partof an electrically resonant LC circuit load where L representsinductance and C represents capacitance. An alternating signal generatedby the external reader 30 is transmitted at various frequencies to‘sweep’ a response from the implant passive circuit. The transmittedinput signal is coupled into the passive circuit at the LC resonantfrequency, ƒ, determined by: $f = {\frac{1}{2\pi}\frac{1}{\sqrt{LC}}}$

[0049] There is a non-ideal resistance, R, in the LC passive circuitthat degrades the resonance response. Along with the membrane deflectionwith pressure, the quality factor, Q, is a measure of the devicesensitivity and is given by: $Q = \frac{2\pi \quad {fL}}{R}$

[0050] The objective is to design the implant circuit 22 with minimumresistance. Coil design, material selection, and interconnection to thepressure sensor are areas where minimal resistance is a critical designparameter.

[0051] If the capacitive membrane 26 is 1 mm×1 mm with a 1 um gap 76,the capacitance is approximately equal to 8.8 picofarads. A realizablemini-inductor can approach 1 microHenry. These values then estimate thatthe electronic detection circuit will operate in the vicinity of 50 mHz.

[0052] Sufficient pressure sensitivity and inductance can be housed inan implanted sensor 20 with dimensions roughly 5 mm in diameter and 0.3mm in thickness. A small die size conflicts with larger membranes andinductor coils for greater sensitivity and lower “tank” frequency.(Inductance is inversely proportional to the square of the frequency.)The sensitivity of the sensor is governed by the flexibility of thecapacitive membrane 26. A thin capacitive membrane 26 of large widthprovide the greatest sensitivity but can lead to nonlinearity problems.This effect is caused by the introduction of tensile stresses in thecapacitive membrane 26 under load. Specialized “bossed” geometries,described above and in FIGS. 4-5, can be implemented for improved linearresponse.

[0053] Careful attention must be made to the electrical properties ofthe sensor structure. Since capacitance change is the measured property,the overall parasitic capacitances, Cp within the system must be kept atreasonable levels to obtain adequate sensitivity. For a capacitivesignal-detecting circuit, the greatest sensitivity is achieved bymaximizing the factor:$\frac{1}{C_{x} + C_{0} + {2C_{p}}}\frac{\partial\left( {C_{x} - C_{0}} \right)}{\partial P}$

[0054] where C_(x) is the capacitor C sensitive to the pressure, P. Thereference capacitor C is designated by C₀. Capacitive membrane 26materials such as highly doped silicon in conjunction with highlyinsulating support layers 80 can effectively limit the parasiticcapacitance.

[0055] One of the key challenges is the accessibility of the blood tothe pressure sensor. Due to the small size of the 3 mm diameter vessels,it is imperative that the implanted sensor 20 be as small as possible inorder to facilitate insertion, minimize flow impedance and preventthrombosis. Thus, the use of the probe 62 to extend into the bloodvessel 12 while leaving the implanted sensor 20 outside the vesselsolves many problems. This approach addresses issues concerning flowimpedance, deployment, retrieval, and arterial embolism due to sensordetachment.

[0056] To avoid occlusion, the tip of the cannula can be capped off witha flexible membrane 74 so that pressure is translated across themembrane to a saline solution column on the opposite side. This designwill communicate the pressure to the sensor external to the artery.

[0057] While the invention has been described with reference to at leastone preferred embodiment, it is to be clearly understood by thoseskilled in the art that the invention is not limited thereto, butincludes all similar, equivalent, or obvious alternatives that could bedevised without undue experimentation by one of reasonable skill in theart.

What is claimed is:
 1. A blood pressure sensor apparatus forperiodically measuring a blood pressure in a blood vessel, the bloodpressure sensor apparatus comprising: an implanted sensor having acapacitor electrically connected to an implant inductor, the implantedsensor being adapted to be positioned adjacent the blood vessel suchthat the capacitor is operatively influenced by the blood pressure inthe blood vessel; an external reader having an external inductor adaptedto be inductively coupled to the implant inductor; and a means fordetermining the blood pressure at the capacitor using the implantinductor and the external inductor.
 2. The blood pressure sensorapparatus of claim 1 wherein the means for determining the bloodpressure includes sweeping the external inductor through a range offrequencies and measuring a dip at a specific frequency, the specificfrequency being determined by the capacitance of the capacitor, which inturn is determined by the blood pressure exerted against the capacitor.3. An implanted sensor for measuring pressure in a conduit through awall, the implanted sensor comprising: a main body having an implantinductor; and a probe with a capacitor electronically connected to theimplant inductor, the probe being adapted to fit through the wall sothat the capacitor can sense pressure in the conduit.
 4. The implantedsensor of claim 3 wherein the probe includes a neck portion that extendsoutwardly to a head portion.
 5. The implanted sensor of claim 4 whereinthe head portion includes a terminus that forms an aperture that iscovered with a flexible membrane that defines an internal chamber. 6.The implanted sensor of claim 5 wherein the capacitor is operativelypositioned adjacent the internal chamber.
 7. The implanted sensor ofclaim 6 wherein the internal chamber is filled with a biocompatiblefluid.
 8. A method for measuring blood pressure, the method comprisingthe steps of: a) providing an implanted sensor having a capacitorelectrically connected to an implant inductor; b) providing an externalreader having an external inductor c) positioning the implanted sensoradjacent the blood vessel such that the capacitor is operativelyinfluenced by the blood pressure in the blood vessel; d) inductivelycoupling the external reader with the implant inductor; e) determiningthe blood pressure at the capacitor using the implant inductor and theexternal inductor.
 9. The method of claim 8 wherein the blood pressureis determined by sweeping the external inductor through a range offrequencies and measuring a dip at a specific frequency, the specificfrequency being determined by the capacitance of the capacitor, which inturn is determined by the blood pressure exerted against the capacitor.